I'm a novice at this....but on my W116 78 300sd the front courtesy light wouldn't work as well. I was doing something else beneath the drivers side steering column and saw a eight or ten prong plug that wasn't plugged in. I placed it into the box on the firewall beneath the steering column area that said HELLA and my front light (over my mirror) my key in ignition and lights on buzzer now work.

Hope this helps.
Duke1
Mike